A co-aperture dual-band linearly polarized slot array based on ridge gap waveguide (RGW), operating at the 74-78- and 102-106-GHz bands, is designed. In each subarray, two groups of longitudinal slots for two frequency bands are excited by a single RGW channel to achieve dual-band unidirectional radiation. Additionally, grooves and notches are implemented inside the RGW structure to improve dual-band matching, and two bandpass filters (BPFs) are incorporated at each side of the subarray to realize independent dual-band feeding while also providing filtering characteristics. The proposed subarray operates as a single channel. It can expand into a multichannel array, enabling multiple-input multiple-output (MIMO) when each subarray is individually excited or broadside radiation when all subarrays are simultaneously excited. A planar array of four subarray columns was fabricated to validate the dual-band concept and its broadside radiation performance. The simulated and measured results show good agreement.

This article presents the design and experimental validation of a W-band waveguide cavity filter operating around 91 GHz, using a periodic electromagnetic bandgap (EBG) structure with glide symmetry to implement the filter in a stack of multiple thin metallic sheets without electric contact between them. The filter is based on vertically stacked cavities that use the TE103 mode, which offers increased robustness to manufacturing and assembly errors due to the large dimensions of the cavity. The glide-symmetric circular hole EBG structure is analyzed and integrated to suppress unwanted field leakage between the metallic layers with a broad stopband. The proposed filter maintains effective operation in the 88-94-GHz frequency range, even with gap variations between layers of up to 20 mu m. The filter is fabricated using laser cutting, achieving a low surface roughness and high dimensional accuracy. Experimental measurements show excellent agreement with the simulations, with a return loss greater than 20 dB and an insertion loss below 0.5 dB. These results demonstrate the possibility to achieve high performance filters at millimeter-wave frequencies while maintaining low fabrication complexity and cost using the multilayer waveguide technology.

This paper presents a Genetic Algorithm (GA)-based methodology for designing high-capacity spectral signature tags for chipless RFID applications. By integrating Frequency Shift Keying (FSK) and Amplitude Shift Keying (ASK), the proposed method enables hybrid-domain encoding, increasing bit capacity without additional resonators. The tags, fabricated using high-resolution screen printing, were experimentally validated through S-parameter measurements, demonstrating strong correlation with simulations. The results show a maximum bit capacity of 12.59 bits, a spectral bit capacity (SBC) of 4.2 bits/GHz, and a spatial bit density (SBD) of $\text{1. 5 7}$ bits $/ \text{c m}^{\text{2}}$, achieving a balance between spectral efficiency and compact design. Comparative analysis with state-of-the-art solutions highlights the method's potential for anticounterfeiting, secure identification, and high-density RFID applications.

This work presents the design of a cavity-stacked bandpass filter (BPF) using contactless air-filled substrate integrated waveguide (CLAF-SIW) technology for millimeter-wave frequencies. This technology is a variant of air-filled SIW technology, incorporating contactless techniques. It enables the reduction of dielectric losses in SIW filters while supporting multilayer structures with robust assembly. The cavity-stacked filter topology allows for very good frequency responses with a reduced footprint and no transitions needed. As an example, a 4th-order Chebyshev bandpass filter composed of four stacked cavities, coupled through irises, is shown. The iris layers are fabricated by metallizing the slot edges of a PCB, while the cavity layers are implemented using CLAF-SIW. The filter has been designed and manufactured to provide a passband response from 36 GHz to 37.5 GHz. A good agreement between measurement and simulation has been achieved. The losses in the proposed CLAF-SIW filter are primarily due to the metal roughness of the low-cost commercial laminates used.

In this contribution, the design and manufacturing of an all-metal coaxial-line X-band bandpass filter is discussed. The device is 3D-printed as a self-supported structure without any dielectric inside the coaxial. The mechanical support between the inner and outer coaxial-line conductors is provided by means of $\lambda /4$ short-circuited stubs, which are also used in the bandpass filter design. The real transmission zeros (TZs) produced by the short-circuited stubs are responsible for a high filter selectivity. In order to enhance the filter performance, a second stage consisting in a coaxial-line stepped-impedance low-pass filter is integrated in the design to provide the rejection level required for the out-of-band behaviour. Following our design method, the bandpass and low-pass filters are designed separately, and a final matching step is performed to connect both and to achieve the aimed frequency specifications. In this way, a monoblock coaxial filter with very good in-band and out-of-band performance can by obtained by using an additive manufacturing (AM) procedure. Only the input/output (I/O) coaxial connectors will need to be assembled to the filter to perform the frequency measurements. The filters in this work can be seen as a first proposal towards more complex multi-functional monoblock structures using additively-manufactured coaxial technology, for highly-integrated RF chains. Other expected benefits beyond the compactness or lightweight are an increased RF shielding, electrostatic discharge risk reduction, and Passive Intermodulation (PIM) protection. In the paper, a prototype with a passband between 8 and 12 GHz is designed and manufactured, using a bandpass filter with three stubs and an integrated 15th-order low-pass filter, providing rejection for spurious frequencies up to 30 GHz. The filter is manufactured using Selective Laser Melting (SLM) and measurements show an excellent agreement with the simulations.

This letter proposes a new design approach for filtering antennas. The novel matching reflection coefficient based method allows the integration of filters and antennas without compromising the frequency behavior of either of these components. Moreover, this integration is done avoiding the need of lengthy optimization processes and provides a high degree of flexibility in the types of antennas that can be used. In order to validate it, two examples are provided. In both cases, a 4 th -order Chebyshev bandpass filter at 101.5 GHz implemented in stacked groove gap waveguide (GGW) configuration is used, firstly along with a single aperture antenna and, subsequently, with a slotted ridge gap waveguide (RGW) array. This second example has been manufactured to demonstrate the usefulness of the new design methodology. Excellent measured performance has been obtained for a filtering antenna at W-band for the first time.

In this paper, a groove gap waveguide (GGW) low-pass filter is proposed for the first time. Gap waveguide technology represents an interesting alternative as a low-loss, cost-effective, high-performance transmission line and packaging solution for microwave and millimeter-wave systems. This technology may exhibit a frequency behavior similar to rectangular waveguide but with some advantages such as the no need of electrical contact between the upper and lower plates of the GGW, making it an attractive alternative in the design of satellite devices at high frequencies. However, all the previous literature focused on band-pass filters, while design methods for GGW low-pass filters have not been reported. Furthermore, in this paper a new manufacturing approach is proposed and its performance has been compared with traditional methods such as Computer Numerical Control (CNC) milling. The new approach relies on the Selective Laser Melting (SLM)-3D printing of the filter followed by a post-processing step, in which it is partially mechanized using CNC milling to improve the surface finish. Measurements of the manufactured prototypes are also included to compare both techniques at millimeter-waves, showing the advantages of the new fabrication method and the excellent agreement with the simulations.

In this article, a novel design method for rectangular waveguide filters intended for fabrication using direct metal additive manufacturing (AM) is proposed. The synthesized filters will feature a smooth profile that allows us to fabricate them orienting the filter propagation axis in the vertical building direction, achieving an optimum configuration for direct metal AM fabrication. The novel design method is valid for any all-pole transfer function, which is initially implemented with a commensurate-line distributed unit element prototype. The impulse response of that initial prototype is then properly interpolated to obtain the target response for a smooth-profiled filter with similar length and profile excursion. Finally, the target impulse response just generated is implemented in the rectangular waveguide technology employing a novel inverse scattering synthesis technique that relies on the coupled-mode theory to model the electromagnetic behavior of the waveguide filter. The novel inverse scattering synthesis technique is general and also valid for the case of filters with very high rejection levels, which is of great relevance in rectangular waveguide technology. A Ku-band low-pass filter with stringent satellite specifications is designed using the proposed method, fabricated by means of a direct metal AM technique, and measured with a vector network analyzer. A very good agreement is achieved between the simulated and measured results, fulfilling the required specifications and demonstrating the feasibility and performance of the novel design method.

In this paper, a design approach for effective low-pass filters (LPF) that use the R th -passband replica of the stepped-impedance prototype response is presented. Traditional LPF design methods often rely on well-established techniques, but they may not always deliver the desired performance or flexibility. By incorporating the R th -passband replica of the stepped-impedance prototype, we introduce a new perspective that opens up new possibilities in filter design. This approach has the potential to overcome some of the limitations associated with existing methods, offering improved filter performance, robustness, and novel design possibilities and flexibility. Additionally, we will showcase its practical application by designing, fabricating, and measuring a 5 th -order gap waveguide LPF that employs the first replica as per the provided guidelines.

A design technique to include multiple and fully-controlled transmission zeros (TZs) in the frequency response of rectangular waveguide commensurate-line stepped-impedance filters is presented in this letter. These bandpass filters (BPFs) are known for having reduced sensitivities against manufacturing inaccuracies and are composed of multiple waveguide sections. In order to improve their selectivity, 3λg/4 and λg/4-stubs are included to create multiple TZs around the passband. The proposed technique allows us to add multiple stubs in a single section and, therefore, only minor adjustments in the affected part of the filter are required, which simplifies the overall design process. The technique has been verified with a design example with four TZs (two on each side) near the passband.

Space Systems have been historically characterised by high performance, high reliability and high cost. Every new generation of space systems tends to improve performance, keep as much as possible reliability, speeding the lead time and lower the cost. Aggressive approach is nowadays followed by some of the players of the new space ecosystem where, for instance, reliability can be relaxed thanks for the in-orbit redundancy or robustness to failures by having a constellation with a high number of satellites. This push towards the technology and system limit requires to investigate new methods for the manufacturing of RF/Microwave parts. RF devices such as those based on waveguide structures, benefit from an additive manufacturing approach in terms of radio frequency (RF) performance and compactness. However each manufacturing approach comes with specific features and limitations which need to be well understood and, in some cases, even taking advantage of them. This paper provides a short review of some of the RF/Microwave parts already manufactured using this technology. The paper will focus mainly on metal 3D printing parts since this technology is, at the moment, well accepted by the space community.

In this paper, a W-band 3D-printed bandpass filter is proposed. The use of higher-order TE 10 n modes in groove gap waveguide (GGW) technology is evaluated in order to alleviate the manufacturing requirements. In addition to the use of higher-order modes, the coupling between them is analyzed in detail to improve the overall fabrication robustness of the component. This allows the implementation of narrow-band filters operating at millimeter-wave frequency bands (or above), which usually demand complex manufacturing techniques to provide the high accuracy required for this kind of devices. In order to show the applicability of the proposed method, a narrow-band 5th-order Chebyshev bandpass filter centered at 94 GHz, which can be easily fabricated by state-of-the-art stereolithographic (SLA) 3D-printing techniques followed by silver coating, is shown. Excellent measured performance has been obtained.

We propose a novel experiment, the Canfranc Axion Detection Experiment (CADEx), to probe dark matter axions with masses in the range 330-460 μeV, within the W-band (80-110 GHz), an unexplored parameter space in the well-motivated dark matter window of Quantum ChromoDynamics (QCD) axions. The experimental design consists of a microwave resonant cavity haloscope in a high static magnetic field coupled to a highly sensitive detecting system based on Kinetic Inductance Detectors via optimized quasi-optics (horns and mirrors). The experiment is in preparation and will be installed in the dilution refrigerator of the Canfranc Underground Laboratory. Sensitivity forecasts for axion detection with CADEx, together with the potential of the experiment to search for dark photons, are presented.